The present invention relates to a casting core composition, and more particularly to a casting core composition which is to be molded by a so-called shell mold process.
A casting core is used for forming internal cavities in a cast product. In fact, a casting core is inserted between two halves of a mold (cope and drag). Then, a molten metal is poured into the mold. After solidification of the metal, the mold is disassembled and then the cast product is removed. After that, the casting core is broken away and removed from the cast product. With this, the cast product will have internal cavities having certain specific shapes.
Nowadays, many casting cores for automobile cast products are produced by the shell mold process. In this process, the casting core is molded out of a mixture of silica sand grains and a thermosetting resin as a binder for binding the silica sand grains. Silica sand contains SiO.sub.2 as a main component thereof. However, if the casting core of this type (silica sand grains bound with a thermosetting resin) is used for casting, for example, an aluminum-alloy automobile cylinder block under a high casting pressure (at least 800 kgf/cm.sub.2), it is necessary to provide the casting core with a certain sufficient strength to withstand the high casting pressure. Silica sand grains themselves have variable polygonal shapes. Thus, a casting core prepared from silica sand grains tend to have spaces between silica sand grains, upon molding of the casting core. With this, the casting core may be broken under the high casting pressure. To prevent this, it is considered to increase the amount of the thermosetting resin to, for example, a range of from 3.5 wt % to 4.2% based on the total weight of the silica sand grains and the thermosetting resin. However, with this, percentage of contraction of the casting core's longitudinally center portion in the direction of the thickness thereof becomes large (for example, 15-17%) after casting, and the amount of a so-called deformation of the casting core's center portion also becomes large (for example, 1.2-1.5 mm) after casting (see the aftermentioned Comparative Example 1). With this, the cast product becomes inferior in dimensional precision. The definition of the amount of this deformation will be explained in detail in the following DESCRIPTION OF THE PREFERRED EMBODIMENTS of this application, with reference to FIG. 6.